The CAA is a regulatory body, and as such it stablishes what we can o cannot do, and how it is to be done , wether you like it or hate it is not what we are talking about, nobody has said so far that being safe (brieffing) and cover your back has to exclude the student wont get pleasure out of the flight, but the rules say if it is a pure pleasure flight it is TRANSPORT, and for that you need an AOC.
As pointed out by others it is in my opinion ok to brief them during the preflight, no need for whiteboard on the first few lesons, and keep the info going in during the flight, but if you don't explain anything untill airborne, chances are the student wont take in any info, because there will be too many things going on when airborne, now, you are suposed to be an FI to do trial lesons, and you should know better than that. After making sure the flight is safe, your first obligation as an instructor is to make sure the student learns.
A good brieffing is of paramount importance no matter what level the student is in.
If you do not think he is going to continue flying, then don't push it to much in the brieffing, keep it simple, but do brieff him/her.